Bellows lid for c4 flip-chip package

ABSTRACT

An electronic package that has a lid that is in direct contact with an integrated circuit that is mounted to a substrate. The lid includes a plate portion that is in contact with the integrated circuit, an outer frame portion that is attached to the substrate, and a bellows portion that couples the plate portion to the outer frame portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic package.

2. Description of Related Art

Present electronic packages typically contain an integrated circuit (IC)that is mounted to a ceramic substrate. The IC is encapsulated by a lidthat is hermetically sealed to the substrate. Because of the tolerancesin the individual components, the packages must be designed so thatthere exist a gap between the lid and the integrated circuit. To improvethe thermal efficiency of the package, a thermal grease is typicallyinserted between the IC and the lid. The thermal grease provides aconductive path between the electrical device and the lid.

The application of thermal grease requires several process steps andmust be contained while the lid is attached to the substrate. Amisapplication of the thermal grease may cause some of the grease toflow in between the integrated circuit and the substrate. The existenceof grease between these two members may degrade the performance of thechip.

Thermal grease also has a low temperature tolerance, such that only lowtemperature adhesives can be used to attach the lid to the substrate. Ithas been found that low temperature adhesives do not always provide acomplete hermetical seal for the package. Additionally, thermal greaseis a relatively poor thermal conductor, such that the package mayproduce unacceptable junction temperatures in the integrated circuit. Itwould therefore be desirable to have an electronic package that isthermally efficient and does not require thermal grease.

SUMMARY OF THE INVENTION

The present invention is an electronic package which has a lid that isin direct contact with an integrated circuit. The circuit is typicallymounted to a substrate that is the lid in a manner that encapsulates theelectrical device. The lid includes a plate portion that is in contactwith the integrated circuit, an outer frame portion that is attached tothe substrate, and a bellows portion that couples the plate portion tothe outer frame portion. The bellows portion provides flexibility in thelid, so that the plate portion is always in contact with the integratedcircuit, regardless of the tolerances in the assembly. The directcontact between the lid and the integrated circuit provides a continuousconductive thermal path that increases the overall thermal efficiency ofthe package.

The outer frame portion of the lid is typically soldered to thesubstrate to hermetically seal the electrical device. The lid ispreferably attached to the substrate so that the plate portion applies apressure to the integrated circuit. The plate pressure creates apre-stress condition on the solder joints between the circuit and thesubstrate. Pre-stressing solder joints has been found to increase thefatigue life of the joints. The plate pressure also decreases thethermal resistance between the lid and the electrical device.

Therefore it is an object of the present invention to provide anelectronic package that is thermally efficient and does not require theapplication of a thermal grease.

It is also an object of the present invention to provide a thermallyefficient electronic package that applies a compressive pre-stress onthe solder joints of the assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become morereadily apparent to those ordinarily skilled in the art after reviewingthe following detailed description and accompanying drawings, wherein:

FIG. 1 is a perspective view of an electronic package of the presentinvention;

FIG. 2 is a cross-sectional view of the electronic package of FIG. 1;

FIG. 3 is a cross-sectional view of an alternate embodiment of theelectronic package of FIG. 1;

FIG. 4 is a cross-sectional view of the electronic package being sealedwith a vacuum collet that has a heating element.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings more particularly by reference numbers, FIGS.1 and 2 show an electronic package 10 of the present invention. Thepackage 10 includes an electrical device 12 that is mounted to asubstrate 14. The electrical device 12 is typically an integratedcircuit which has a plurality of surface pads that are soldered tosurface pads on the substrate 14. Although an integrated circuit isdescribed, it is to be understood that the electrical device can be anytype of electrical component. The substrate 14 is typically constructedfrom a ceramic material that contains conductive lines that couple theintegrated circuit 12 to a plurality of pins 16 located on the bottom ofthe substrate 14. The pins 16 are typically soldered to a printedcircuit board (not shown) as is known in the art.

The electrical device 12 is encapsulated by a lid 18 that is attached tothe substrate 14. The lid 18 has a plate portion 20 that is adjacent toa bottom surface of the device 12, an outer frame portion 22 that isattached to the substrate 14 and a bellows portion 24 that couples theplate portion 20 to the outer frame portion 22. The lid 18 is typicallyconstructed from a thermally conductive material such as aluminum. Theplate 20 and frame 22 portions are constructed to be flat and rigid. Thebellows portion 24 is constructed to be flexible.

The lid 18 can be constructed from a single sheet of aluminum, whereinthe bellows portion 24 is formed by stretching and crimping thematerial. Although creating the lid 18 from a single sheet is described,it to be understood that the lid 18 can be constructed from multiplepieces, which may or may not be formed from the same material.Additionally, although a flat outer frame portion 22 is shown anddescribed, it is to be understood that the bellows portion 24 could beattached directly to the substrate 14.

The lid 18 is attached to the substrate 14 so that the plate portion 20is always contiguous with the electrical device 12. The direct contactbetween the plate portion 20 and the integrated circuit 12 reduces thethermal resistance between the two members and the junction temperaturesof the circuit 12. As shown in FIG. 3, the lid 18 may have a pluralityof fins 26 that improve the overall thermal efficiency of the lid 18.The fins 26 may be soldered or brazed to the plate portion 20 todecrease the thermal resistance of the thermal interface between theheat sink 26 and the lid 18.

The outer frame portion 22 may be attached to the substrate 14 by asolder 28. The circuit 12 is typically hermetically sealed in an inertgas by the lid 18, the substrate 14 and the solder 28. Although a solder28 is described and shown, it is to be understood that the lid 18 may beattached to the substrate 14 by gold or an adhesive such as an epoxy.

The lid 18 is preferably constructed so that the plate portion 20applies a pressure to the electrical device 12 to insure that the device12 is always in contact with the lid 18. The lid pressure also providesa compressive pre-stress on the solder joints between the integratedcircuit 12 and the substrate 14. Compressive pre-stresses have beenfound to increase the fatigue life of the solder joints. The lidpressure also reduces the thermal resistance between the plate portion20 and the device 12.

The bellows portion 24 is typically constructed to be rigid enough toapply a pressure to the electrical device 12, yet flexible enough tocompensate for tolerances in the assembly 10. For example, if thecomponents 12, 14 and 18 are oversized, the bellows portion 24 willexpand during assembly to insure contact between the lid 18 and thedevice 12. In the preferred embodiment, the lid 18 is constructed sothat the bellows portion 24 expands during the assembly of every package10. Such expansion insures that the plate portion 20 is always incontact with the electrical device 12 regardless of the componenttolerances. The bellows portion 24 also compensates for any unequalthermal expansion between the lid 18 and the substrate 14, that mayresult when the members have different coefficient of thermalexpansions. Compensating for the different thermal expansion rates,reduces the stresses in the solder 28 and improves the overall life ofthe package 10.

FIG. 4 shows a preferred method of attaching the lid 18 to the substrate14. A solder paste 28 is initially applied to the substrate 14 by atemplate (not shown). The lid 18 is held by a vacuum collet 30. Thecollet 30 includes a housing 32 with a passage 34 that is coupled to avacuum source (not shown) such as a compressor. The vacuum sourcecreates a vacuum between the housing 32 and the lid 18 to hold the lid18 in place.

The lid 18 is placed onto the substrate 14 so that the outer frameportion 22 is adjacent to the solder paste 28. The outer frame portion22 is also adjacent to an annular arm 36 which contains a heatingelement 38. The heating element 38 provides a local supply of heat tothe outer frame portion 22, to elevate the temperature of the solderpaste 28. The heating element is held in place until the solder pasteliquefies. After a predetermined amount of time, the vacuum between thelid 18 and housing 32 is released and the collet 30 is removed from thepackage 10. The same process can be used when gold or an adhesive isused to attach the lid 18 to the substrate 14. As an alternate methodthe entire package may be heated to reflow the solder, etc.

Although the lid 18 is described and shown as being attached to asubstrate 14, it is to be understood that the electrical device 12 andthe lid 18 could be coupled directly to a printed circuit board. Thecollet 30 could then be part of a pick and place machine that attachesthe lids 18 to the board in accordance with a predetermined program.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

What is claimed is:
 1. An electronic package, comprising:a substrate; anelectrical device coupled to said substrate; and, a lid that has a plateportion contiguous with said electrical device, an outer frame portionattached to said substrate and a bellows portion coupled to said outerframe portion and said plate portion, said lid being attached to saidsubstrate so that said plate portion applies a pressure to saidelectrical device, said bellows portion extending between said plateportion and said outer frame portion in both an axial and longitudinaldirection relative to said substrate.
 2. The package as recited in claim1, wherein said outer frame portion is attached to said substrate bysolder.
 3. The package as recited in claim 1, wherein said lid includesa plurality of fins extending form said plate portion.
 4. The package asrecited in claim 1, wherein said lid is constructed from aluminum. 5.The package as recited in claim 1, wherein said outer frame portion isattached to said substrate by epoxy.
 6. The package as recited in claim1, wherein said outer frame portion is attached to said substrate bygold.
 7. A method for assembling an electronic package, comprising thesteps of;a) providing an electrical device that is mounted to asubstrate; b) placing a lid adjacent to said electrical device so that aplate portion of said lid is contiguous with said electrical device andan outer frame portion of said lid is adjacent to said substrate, saidplate portion being separated from said outer frame portion by a bellowsportion which extends in both an axial and longitudinal directionrelative to said substrate; and, c) coupling said outer frame portion tosaid substrate.
 8. The method as recited in claim 7, wherein said lid isattached to said substrate by heating said lid and said substrate.